1. Field of the Invention
The present invention relates to a radiation image detecting device, an electronic cassette, and a radiation imaging system having an automatic exposure control function.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays is widely known. The X-ray imaging system is constituted of an X-ray source for emitting the X-rays to an object, an X-ray image detecting device for detecting an X-ray image formed by the X-rays passed through the object, a control device for controlling the operation of the X-ray source and the X-ray image detecting device, and an emission switch for issuing an emission start command of the X-rays.
In the field of the X-ray imaging system, recently, the X-ray image detecting device that uses a flat panel detector (FPD) as a detection panel, instead of an X-ray film or an imaging plate (IP), becomes widespread. The FPD has a matrix of pixels each of which accumulates signal charge in accordance with an X-ray dose incident thereon. The FPD converts the signal charge accumulated in the individual pixels into a voltage signal, and outputs the X-ray image of the object as a time-series signal.
An electronic cassette (portable X-ray image detecting device) that has the FPD contained in a rectangular parallelepiped housing is in practical use. The electronic cassette is mounted for use not only on a specific imaging stand, but also on an existing imaging stand shareable with a film cassette and an IP cassette. Furthermore, the electronic cassette is sometimes used while being put on a bed under a patient's body or held by the patient himself/herself to take a radiograph of a body portion that is hard to handle with a stationary type of X-ray image detecting device. Also, the electronic cassette is sometimes brought out from a hospital for use in bedside radiography of a home-care patient or in an outside accident or natural disaster site having no imaging stand in case of emergency.
The X-ray image detected by the electronic cassette is transmitted to the control device, and written to a memory. This transmission is performed through wired communication. The imaging stand specific to the electronic cassette is provided with a connector in its cassette holder. By loading the electronic cassette in the imaging stand, a socket of the electronic cassette is connected to the connector. This connector is connected to the control device via a cable extending through the interior of the imaging stand. On the other hand, in the case of loading the electronic cassette in the existing imaging stand designed for the film cassette, or in the case of using the electronic cassette being put on the bed, the electronic cassette is connected to the control device through an external cable.
In the case of establishing the wired communication through the external cable connected between the electronic cassette and the control device, the cable becomes a nuisance. Thus, an electronic cassette having a wireless communication function is developed too. Wireless communication, however, is inferior to the wired communication in transmission speed. Worse yet, the wireless communication is in danger of causing interference from communication radio waves with other electronic equipment, so the electronic cassette having the wireless communication function is hard to use for the patient who has a pacemaker implanted in his/her heart. Therefore, as described in Japanese Patent Laid-Open Publication No. 2008-154721, an electronic cassette having both the wireless communication function and the wired communication function is developed.
The electronic cassette having both the wireless communication function and the wired communication function contains a battery used as a power source in the wireless communication. In the wired communication, the battery is recharged while the electronic cassette receives power supply from the control device. Since the wired communication is superior to the wireless communication in resistance to communication failure, the electronic cassette is automatically switched to the wired communication upon detecting the connection of the cable directly or through the cassette holder.
Also, the electronic cassette having an automatic exposure control (AEC) function is widely known. This electronic cassette is provided with a dose detection sensor for detecting the X-ray dose passed through the object. When an integral value of the detected X-ray dose i.e. an integral dose has reached a predetermined value, or when exposure time corresponding to the predetermined integral dose has elapsed, the X-ray emission from the X-ray source is stopped, and the FPD is shifted from a charge accumulation operation to a readout operation.
Japanese Patent Laid-Open Publication No. 2002-000590 describes an X-ray image detecting device that uses some of the pixels as the dose detection sensors. In this device, the X-ray dose is measured based on a signal from the dose detection sensor, and a signal (emission stop signal) for commanding the stop of X-ray emission is issued. The emission stop signal is transmitted to the control device through the wired communication.
The wired communication has various merits, as described above. However, in case of the communication failure, the wired communication requires a long time to find out a cause due to many relay parts including the cable and connectors. To be more specific, a cable break, a connector contact failure, a breakdown in a relay device such as a hub, a breakdown in another device connected through a network, and the like are conceivable as the cause of the wired communication failure. In the occurrence of the communication failure, the cable break, the connector contact failure, and the operation of the relay device are checked to identify the cause, and it requires a long time. Furthermore, after the identification of the cause, a broken part needs replacing and a replaced new part needs operation check, so it requires a longer time.
In the Japanese Patent Laid-Open Publication Nos. 2002-000590 and 2008-154721, an AEC signal is transmitted through the wired communication. In such systems, recovery in the communication of the AEC signal is absolutely necessary for radiography, and delay in the recovery elongates downtime of the system. However, neither of the Japanese Patent Laid-Open Publication Nos. 2002-000590 and 2008-154721 describes an object of rapidly recovering from the communication failure of the AEC signal and a solution thereof.